[uml-user] Byte count discrepency in ifconfig

[David C. <da...@da...>]

I switched my UMLs over to use a kernel bridge, rather than uml_switch, 
so I've got a bunch of tap devices sitting on the host for connectivity 
to the UML. However, there seems to be some discrepency between the TX 
bytes on the tap device, and the RX bytes on the UML.

uml-web0  Link encap:Ethernet  HWaddr 00:FF:6C:6D:70:36
           inet6 addr: fe80::2ff:6cff:fe6d:7036/10 Scope:Link
           UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
           RX packets:1411334 errors:0 dropped:0 overruns:0 frame:0
           TX packets:1361146 errors:0 dropped:1 overruns:0 carrier:0
           collisions:0 txqueuelen:100
           RX bytes:262129828 (249.9 MiB)  TX bytes:246860369 (235.4 MiB)

eth0      Link encap:Ethernet  HWaddr FE:FD:0A:01:07:05
           inet addr:10.1.7.5  Bcast:10.255.255.255  Mask:255.255.255.0
           inet6 addr: fe80::fcfd:aff:fe01:705/10 Scope:Link
           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
           RX packets:1361150 errors:0 dropped:0 overruns:0 frame:0
           TX packets:1411336 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:100
           RX bytes:227804597 (217.2 MiB)  TX bytes:262130024 (249.9 MiB)
           Interrupt:5


The number of packets is near enough identical, but there's a 
significant difference (~7%) between bandwidth usage of the tap device, 
and the interface on the UML side. Does anyone have an idea what 
could/does account for this? I'm intending setting up SNMP bandwidth 
accounting, but if the values of the host tap device don't match that of 
the UML, then people are going to complain. The RX on the host is the 
same as the TX on the UML, so I'm sort of confused as to why that's 
right, when bandwidth usage of packets to the UML isn't.

All my other UMLs on the host exhibit the same effect.

David

-- 
David Coulson                                  http://davidcoulson.net/
d...@vi...                       http://journal.davidcoulson.net/